Conventionally, exhaust combustion gas, which is exhausted from a boiler that burns coal or heavy oil, includes components such as NOx, SOx, CO2, and CO. And a gas analysis apparatus has been developed that analyzes the contents of the components in the gas. As such a gas analysis apparatus, for example, an apparatus employing probe type has been developed. According to the gas analysis apparatus of probe type, measurement light is emitted from a light source, and the measurement light is reflected by a reflector arranged at a tip end of the probe. The apparatus analyzes constituent concentration of the sample gas based on information on the measurement light reflected by the reflector.
Some of the conventional gas analysis apparatus of probe type include zero correction function as well as the above-described component concentration analysis function. For example, one measurement light, which is emitted from the light source, is branched into two beams by an optical coupler and splitter, and one of the beams is used for analyzing the constituent concentration of the gas, and the other of the beams is used for the zero correction. The one beam used for analysis on constituent concentration of the gas and the beam used for the zero correction are input into different light receiving units and the signals are processed individually.
However, the above-described gas analysis apparatus includes problems below. The optical coupler and splitter have wavelength dependence, and cannot output the two beams in the same intensity after branching, depending on the wavelength band. In addition, the light receiving units have individual differences (differences among the devices) too, and the outputs may be actually different from each other in many cases, even though the light receiving units based on the same design are primarily intended to generate the same outputs in response to the same inputs. In addition, different signal processing units, which receive output from the light receiving unit, are employed for different light receiving units, so that the processing results by the signal processing units have some individual differences. Accordingly, in the signal processing result obtained based on the above-described two beams after the branch, there is a great chance of a difference resulting from the accumulation of each of the individual differences among the parts. Accordingly, it is impossible to accurately perform the zero correction, so that it is difficult to perform a highly accurate components analysis, which is a problem. Furthermore, it is necessary to have different systems (systems having a light receiving unit and a signal processing unit) for the two beams after the branch, so the whole analysis apparatus has to grow in size. In addition, the heating of the systems increases the heating value of the analysis apparatus as a whole, and the durability of the signal processing circuit is deteriorated, which is a problem. Since the gas analysis apparatus of probe type is attached to the flue for use, it is likely affected by heat of the sample gas and gets higher in temperature, so that it is likely to be deteriorated.
Furthermore, as a gas analysis apparatus of probe type, one that has calibration function as well as component concentration analysis function is disclosed in Patent Document 1. The gas analysis apparatus disclosed in Patent Document 1 includes a probe tube formed with an introduction hole through which the sample gas is introduced. According to the probe tube, most of its parts, including the tip end portion, is positioned inside of a gas flue wall (on a side of the gas flue), and only a base end portion is positioned outside the gas flue wall (on an opposite side of the gas flue). According to this gas analysis apparatus, the measurement light is emitted from the light source positioned outside the gas flue wall toward the sample gas in the probe tube. The measurement light is reflected by a first reflector arranged at a tip end portion of the tubular housing, and the reflected measurement light is received by a light receiving sensor arranged outside the gas flue wall. Based on information on the measurement light obtained at the light receiving sensor, the concentration of certain components contained in the sample gas can be calculated.
This gas analysis apparatus includes, as described above, a function of reflecting the measurement light emitted from the light source at the first reflector and analyzing the constituent concentration of the sample gas. The gas analysis apparatus further includes a function of reflecting the measurement light emitted from the light source at the second reflector and calibrating the gas analysis apparatus. The second reflector is positioned at a middle portion of the probe tube and inside of the gas flue wall. The position of the second reflector can be changed by a switching unit. The switching unit is positioned in a middle portion of the probe tube and inside of the gas flue wall, and is configured to move the second reflector out of a light path when analyzing the component concentration and to place the second reflector into the light path when performing the calibration. According to the switching operation by the switching unit, it is possible to selectively perform the analysis of constituent concentration of the gas and the calibration for the gas analysis apparatus.